1. Field of the Invention
The present invention relates to an LED AC driving circuit, particularly an LED AC driving circuit capable of adjusting an operating voltage.
2. Description of the Prior Art
LED lights as the popular and common lighting devices have the advantages of high illumination efficiency and low power consumption in comparison to the incandescent light. Limited by the one-way driving characteristic of the LED, the LED lights cannot be connected to AC sockets directly. One of the conventional LED AC driving circuits as shown in FIG. 9 is proposed to overcome the limitation.
The LED AC driving circuit comprises a rectifying unit 20, an LED unit 21, a voltage-controlled transistor 22, a current detecting unit 23, a low frequency filter 24 and a current regulating unit 25.
The rectifying unit 20 receives and converts AC power to pulsed DC power. The LED unit 21 is comprised of multiple LEDs and connected to the rectifying unit 20 to form a power loop. The voltage-controlled transistor 22 is connected in the power loop for adjusting a loop current flowing through the power loop. The current detecting unit 23 is connected in the power loop for converting the loop current to a voltage signal. The low-frequency filter 24 electrically connects to the current detecting unit 23 and outputs a voltage based on the voltage signal from the current detecting unit 23. The current regulating unit 25 has input terminals connecting to low-frequency filter 24 and a reference voltage (Vref), and an output connecting to the voltage-controlled transistor 22. The current regulating unit 25 outputs a control signal to the voltage-controlled transistor 22 according to a comparison result of the reference voltage (Vref) and the voltage for stabilizing the loop current.
In short, the rectifying unit 20 converts the AC power, which cannot be directly applied to the LED, to pulsed DC power. The current detecting unit 23 and the low frequency filter 24 detect the loop current flowing through the LED unit 21. The current regulating unit 25 controls the voltage-controlled transistor 22 to adjust the loop current for maintaining the normal operation of the LED unit 21.
However, the voltage-controlled transistor 22 may encounter the problem of over-heating because it is connected in the power loop and sustains a large power. Therefore, a shunt resistor 26 can be connected in parallel to the voltage-controlled transistor 22 to solve the problem.
With further reference to FIGS. 10 and 11, although the shunt resistor 26 is able to share the power of the voltage-controlled transistor 22 to mitigate the extent of heat accumulation, the shunt resistor 26 in another aspect limits the maximum operating voltage across the voltage-controlled transistor 22. As an example, if the loop current is 0.16 ampere and the rated power of the voltage-controlled transistor 22 is 1 watt, the maximum operating voltage across the voltage-controlled transistor 22 will be limited at 24 volts when the resistance of the shunt resistor 26 is 150 ohms. However, when the operating voltage of the voltage-controlled transistor 22 needs to be increased over 24 volts, the current passing through the voltage-controlled transistor 22 will become zero. The voltage-controlled transistor 22 eventually loses its current regulating function. Therefore, it seems that the conventional AC LED driving circuit is unsuitable for particular applications with high operating voltages.
To overcome the shortcomings, the present invention provides an LED AC driving circuit capable of adjusting the operating voltage to mitigate or obviate the aforementioned problems.